1. Field of the Invention
The present invention relates to a transmission signal processing apparatus for use in a modem, for performing a transmission signal process consisting of a roll-off filtering process for limiting the band of a transmission point signal and an interpolation process for raising the sampling frequency of the transmission point signal to a frequency at which the transmission point signal can be modulated.
2. Description of the Related Art
In analog voice band transmission lines such as public telephone transmission lines and dedicated transmission lines, modems (modulation and demodulation units) are being widely used.
In recent years, digital signal processing for encoding and decoding in a modem have been executed by a DSP (Digital Signal Processor) and a MPU (Micro Processing Unit) so as to reduce the size and cost of the modems.
FIGS. 1A, 1B, and 2A to 2D are schematic diagrams for explaining a conventional transmission signal processing portion for use in a model according to a related art reference.
In the conventional transmission signal process, as shown in FIG. 1A, transmission data is encoded and converted into a transmission point signal. Thereafter, the frequency band of the transmission point signal is limited by a roll-off filtering process executed by a roll-off filter portion 1. The output of the roll-off filter portion 1 is modulated by a modulation portion 2 and thereby a modulated transmission signal is generated.
The roll-off filtering process is a process for limiting the band of a transmission point signal which is an impulse signal without an occurrence of inter-symbol interference so as to allow the signal to travel over a transmission line. This signal process is important and well known as a data transmission technique.
Recently, modems use a frequency division modulation (FDM) system for dividing the frequency band of one transmission signal to be transmitted over a transmission line into a plurality of subbands and for transmitting a plurality of transmission point signal channels with the plurality of subbands.
As an example shown in FIG. 1B, a transmission point signal with a transmission rate of 9600 bps (bits per second) on channel 1 and transmission signals with a transmission rate of 2400 bps on channels 2 and 3 are multiplexed in an analog voice band with a frequency characteristic ranging from 0.3 to 3.4 kHz.
FIG. 2A shows an example of the construction of a conventional transmission signal processing portion which accomplishes the frequency division modulation shown in FIG. 1B.
Now, assume that an encoding system for converting data of five bits into a transmission point signal is used. In this case, when data with a transmission rate of 9600 bps is converted, the modulation frequency (sampling frequency) of a transmission point signal becomes 1920 Hz (because 9600 Hz/5=1920 Hz).
A roll-off filter portion 1a limits the frequency band of the transmission point signal and performs an interpolation process with a multiple of four. Thus, the sampling frequency (1920 Hz) of the transmission point signal is raised to a frequency equal to the sampling frequency of a transmission signal which is transmitted over a transmission line (this sampling frequency is for example 7680 Hz). In other words, three new samples (4-1=3) with a value of 0 are interpolated or inserted between each sample of the transmission point signal being received. Thus, an output transmission point signal whose sampling frequency is 4 times higher than that of an input transmission point signal is obtained. Then, the above-mentioned roll-off filtering process is performed on the output transmission point signal. Thus, the sampling frequency of the transmission point signal is raised while the frequency component of the original transmission data are kept.
Generally, an interpolation process with a multiple of n is a process for interpolating (n-1) new samples with a value of 0 between each adjacent sample of an input transmission point signal.
The transmission point signal with a frequency of 7680 Hz which is equal to the sampling frequency of the transmission signal which is transmitted over the transmission line is modulated by a modulation portion 2a. Thus, a pass band signal with the frequency characteristic of the channel 1 (shown in FIG. 1B) is obtained.
On the other hand, when the data with the transmission rate of 2400 bps is transformed, the modulation frequency of the resultant transmission point signal becomes 480 Hz (because 2400 Hz/5=480 Hz).
In the sample shown in FIG. 2A, there are two transmission point signal channel with this sampling frequency because there are two signal transmission sources.
Roll-off filter portions 1b and 1c each limit the frequency band of the corresponding channel of the transmission point signal and perform an interpolation process with a multiple of 16 for the signal. Thus, the sampling frequency (480 Hz) of the input signal is raised to the sampling frequency of the transmission signal which is transmitted over the transmission line (the resultant sampling frequency is for example 7680 Hz). In this case, 15 new samples with a value of 0 (because of 16-1=15) are interpolated between each adjacent sample of the input point signal.
The two series of the transmission point signals are sent to modulation portions 2b and 2c, respectively. The modulation portions 2b and 2c modulate the two series of the transmission point signals, respectively. Thus, two channels of pass band signals with the frequency characteristic of the channels 2 and 3 shown in FIG. 1B are obtained.
Thereafter, the output pass band signals of the modulation portions 2b and 2c are sent to an addition portion 4a. The addition portion 4a adds these two pass band signals. The output pass band signal of the modulation portion 2a and the output pass band signal of the addition portion 4a are sent to an addition portion 4b. The addition portion 4b adds these pass band signals. The addition result is sent to a D/A (Digital to Analog) converter (not shown in the figure). The D/A converter converts the input signal into a modulated analog transmission signal and outputs the resultant signal to the transmission line.
As shown in FIG. 2B, the roll-off filter portions 1a, 1b, and 1c each has the construction of a transversal filter which comprises a plurality of taps, a plurality of multiplication portions, and an addition portion. The taps are connected in a cascade configuration. The output of each tap is connected to one of the multiplication portions. Each multiplication portion multiplies the output of a tap by a tap coefficient. The output of each multiplication portion is sent to the addition potion. The addition portion adds all the outputs of the multiplication portions and outputs the addition result as an output z.
When the interpolation process with a multiple of n is executed, (n-1) samples with a value of 0 are interpolated between each adjacent sample of an original transmission point signal. Thereafter, the resultant signal is sent to the roll-off filter portion 1 at a sampling period T/n (where T is the sampling period before the interpolation process is executed). The resultant signal is sent to the taps which are connected in the cascade configuration, each tap having a delay time of T/n.
In this case, as described above, in the interpolation process with a multiple of n, although one sample of n successive samples of the input signal has the value of an original transmission point signal, other samples thereof, (n-1) samples, have a value of 0.
Thus, at a given time, only tap outputs d.sub.1, d.sub.n+1, . . . , d.sub.(k-2)n+1, and d.sub.(k-1)n+1 disposed at intervals of n taps apart have the values of the original transmission point signal. The values of other taps are 0. As a result, at this time, the output z can be given by the following formula. EQU z=C.sub.1 d.sub.1 +C.sub.n+1 d.sub.n+1 + . . . +C.sub.(k-2)n+1 d.sub.(k-2)n+1 +C.sub.(k-1)n+1 d.sub.(k-1)n+1 ( 1)
Next, when a time T/N has elapsed from the above timing, the sample stored at each tap is moved to the tap immediately to the right. Thus, only the tap outputs d.sub.2, d.sub.n+2, . . . , and d.sub.(k-2)n+2 have the value of the original transmission point signal. The values of other tap outputs are 0. Thus, the output z at this time can be given by the following formula. EQU z=C.sub.2 d.sub.2 +C.sub.n+2 d.sub.n+2 + . . . +C.sub.(k-2)n+2 d.sub.(k-2)n+2 ( 2)
Likewise, after every T/n time period, the position of each tap output with the value of the original transmission point signal moves one place to the right. When a {(n-1).times.T/n} time period elapses from the first point in time, only the tap outputs d.sub.n, d.sub.2n, . . . , and d.sub.(k-1)n which are disposed at intervals of n taps apart have the values of the original transmission point signal. The values of other tap outputs are 0. thus, the output z at this time can be given by the following formula. EQU z=C.sub.n d.sub.n +C.sub.2n d.sub.2n + . . . +C.sub.(k-1)n d.sub.(k-1)n ( 3)
After a {n.times.T/n} time period has elapsed from the first point in time, the tap outputs d.sub.1, d.sub.n+1, . . . , d.sub.(k-2)n+1, and d.sub.(k-1)n+1 which are disposed at intervals of n taps apart have the value of the original transmission point signal. Thus, the output z is also given the formula (1).
According to the formulas (1) to (3), until a {(n-1).times.T/n} time period elapses from the first timing, tap outputs of n successive taps d.sub.1 to d.sub.n have the value of the original transmission point signal. Likewise, tap outputs of n successive taps d.sub.n+1 to d.sub.2n, . . . , and d.sub.(k-2)n+1 to d.sub.(k-1)n have the same values, respectively.
Thus, the (k-1)n+1 taps with a delay time of T/n shown in FIG. 2B can be substituted with k taps with a delay time of T shown in FIG. 2C. When the original transmission point signal with a sampling period of T is inputted to the roll-off filter portion 1, the roll-off filter portion 1 can execute both the interpolation process with a multiple of n and roll-off filtering process at the same time.
In other words, as shown in FIG. 2C, in a predetermined timing t.sub.m, k tap outputs d.sub.1 to d.sub.k are multiplied by k tap coefficients C.sub.1, C.sub.n+1, . . . , C.sub.(k-2)n+1, and C.sub.(k-1)n+1 which are disposed at intervals of n taps aparts, respectively, and thereby an output z is obtained. The output z is given by the following formula. EQU z=C.sub.1 d.sub.1 +C.sub.n+1 d.sub.2 + . . . +C.sub.(k-2)n+1 d.sub.k-1 +C.sub.(k-1)n+1 d.sub.k ( 4)
Next, in a timing t.sub.m+T/n when a T/n time elapses from a timing t.sub.m, (k-1) tap outputs d.sub.1 to d.sub.k (except for the last tap output) d.sub.k-1 are multiplies by (K-1) tap coefficients C.sub.2, C.sub.n+2, . . . , and C.sub.(k-2)n+2 which are disposed at just right positions of (k-1) tap coefficients C.sub.1, C.sub.n+1, . . . , and C.sub.(k-2)n+1, respectively, and thereby an output z is obtained. The output z is given by the following formula. EQU z=C.sub.2 d.sub.1 +C.sub.n+2 d.sub.2 + . . . +C.sub.(k-2)n+2 d.sub.k-1 ( 5)
Likewise, whenever a T/n time elapses, (k-1) tap outputs d.sub.1 to d.sub.k which are the same as those in the first timing t.sub.m are multiplied by (k-1) tap coefficients which are disposed at just right positions thereof, respectively. Thus, an output z is obtained. In the timing t.sub.m+(n-1)T/N when a {(n-1).times.T/n} time elapses from the first timing t.sub.m, (k-1) tap outputs d.sub.1 to d.sub.k which are the same as those in the first timing are multiplied by (k-1) tap coefficients C.sub.n, C.sub.2n, . . . , and C.sub.(k-1)n, respectively, and thereby an output z is obtained. The output z is given by the following formula. EQU z=C.sub.n d.sub.1 +C.sub.2n d.sub.2 + . . . +C.sub.(k-1)n d.sub.k-1 ( 6)
In a timing t.sub.m+T when a T time (namely, n.times.T/n=T) elapses from the first timing t.sub.m, according to the formula (4), (k-1) tap outputs d.sub.1 to d.sub.k whose values differ from those in the first timing t.sub.m are multiplied by k tap coefficients C.sub.1, C.sub.n+1, . . . , C.sub.(k-2)n+1 and C.sub.(k-1)n+1, respectively, and thereby an output z is obtained.
According to the construction of the roll-off filter portion 1 shown in FIG. 2C, the construction of the roll-off filter portions 1b and 1c which execute the interpolation process with a multiple of 16 shown in FIG. 2A can be represented by a construction shown in FIG. 2D. FIG. 2D shows the construction of FIG. 2C where n=16 and k=63 (where n is a multiple of interpolation process; and k is the number of taps).
In this case, the roll-off filter portion comprises 63 taps 10-1 to 10-63, 63 multiplication portions 11-1 to 11-63, and an addition portion 12. Thus, the number of tap coefficients C.sub.1 to C.sub.(k-1)n+1 (namely, C.sub.1 to C.sub.993) is 993.
When the transmission signal process is executed by the DSP as a digital signal process, the taps of the roll-off filter portion 1 are constructed of a RAM (Random Access Memory). The tap coefficients are stored in a ROM (Read Only Memory). The multiplications and additions are executed by an arithmetic and logic unit in the DSP 2.
Since the transmission point signal is inputted as real component REFX and imaginary component REFY, the roll-off filter portion 1 separately processes them.
However, the above-mentioned related art reference as the following problems.
To increase the data transmission rate on a transmission line with a limited band width, the modulation rate should be raised. In particular, when a plurality of channels of transmission point signals are multiplexed according to the FDM system as shown in FIG. 1B, the amount of attenuation of unnecessary band components of each channel should be increased so as to obtain a high S/N ratio. Thus, the transmission point signals should not generate an inter-symbol interference. To do that, in the frequency spectrum characteristic of the roll-off filter portion 1, the roll-off ratio of the frequency band of the cut-off region to the entire frequency band width should be lowered so that the cut-off characteristic becomes sharp. As a result, the convergence time of the time region response waveform of the roll-off filter portion 1 becomes long, resulting in an increase of the number of taps k of FIG. 2C.
When the multiple n of the interpolation process of the roll-off filter portion 1 becomes large such as n=16 (as shown in FIG. 2D), the number of groups of the tap coefficients becomes 16. Thus, when the number of taps k becomes large such as k=63 (as shown in FIG. 2D), the number of tap coefficients (k-1)n+1 increases to such as 993 (as shown in FIG. 2D).
When tap coefficients of the roll-off filter portion 1 are selected so that the frequency spectrum characteristic thereof has a special shape referred to as the cosine roll-off shape, some values of tap coefficients of each group become 0. For these tap coefficients, since the multiplications of the tap outputs are not required, the amount of arithmetic operations can be reduced. However, when the multiple n of the interpolation process is high such as n=16 (as shown in FIG. 2D), many samples (16 samples in the case shown in FIG. 2D) are interpolated between each adjacent sample of the time region response waveform of the roll-off filter portion 1. Thus, even if the above-mentioned special response waveform is used, the tap coefficients with a value of 0 are at most one every 16. Thus, the amount of arithmetic operations cannot be reduced.
As described above, according to the related art reference, in the DSP, both the capacity of the ROM which stores the tap coefficients and the amount of arithmetic operation increase. Thus, it is necessary to extend a processor, an external RAM device, and so forth. Thus, the circuit scale increases, resulting in preventing the size and cost of the apparatus from being reduced.